1. Field of the Invention
The device of the present invention generally relates to nuclear fuel assemblies for use with nuclear reactors, and more particularly, to a new and more improved cruciform skeleton and water cross for a water flow channel of a nuclear fuel assembly especially adapted for use in a boiling water reactor (BWR).
2. Description of the Prior Art
The generation of a large amount of heat energy through nuclear fission in a nuclear reactor is well known. This energy is dissipated as heat in elongated nuclear fuel rods. A plurality of nuclear fuel rods are usually grouped together to form separately removable nuclear fuel assemblies. A number of such nuclear fuel assemblies are typically arranged in a matrix to form a nuclear reactor core capable of a self sustained, nuclear fission reaction. The core is typically submersed in a fluid, such as light water, that serves as a coolant for removing heat from the nuclear fuel rods and as a neutron moderator.
A typical nuclear fuel assembly may be formed by a 7.times.7 or 8.times.8 array of spaced apart, elongated rods supported between upper and lower tie plates. Examples of such nuclear fuel assemblies are depicted and described in U.S. Pat. Nos. 3,350,275; 3,466,226; 3,802,995. In a typical boiling water reactor a nuclear fuel assembly having an 8.times.8 rod array, the sixty four rods that form the 8.times.8 array may be either sixty four fuel rods or may have one or more non-fueled, water moderator rods with remaining rods being fuel rods. A common problem in typical boiling water reactor nuclear fuel assemblies of the types depicted in the above identified patents is that the central region of the fuel assemblies may be under-moderated and over-enriched. In order to increase the flow of moderator, one or more elongated, water moderator rods have been substituted for fuel rods in the central region of such nuclear fuel assemblies. For example, water moderator rods 41 and 42 are depicted in and described in the above identified '995 patent. Of course, the use of one or more moderator rods prevents the use of a full compliment of fuel rods, i.e. one fuel rod in each rod lattice position within the fuel assembly.
In the above mentioned typical boiling water reactor nuclear fuel assembly, the outer flow channel has been formed of an alloy of zirconium known as Zircaloy and is structurally unsupported across its cross section perpendicular to its longitudinal axis. At reactor operating conditions, such an outer flow channel may deform slightly due to creep. Attempts have been made to counteract creep deformation by using an outer flow channel with thicker walls in order to extend its life. However, using thicker walls increases parasitic neutron absorption in the reactor core with resultant increases in fuel cycle costs.
In some BWR fuel assemblies an elongated internal central water cross is used to eliminate the need for water moderator rods, thus allowing a full compliment of fuel rods to be used, that is one fuel rod in each rod lattice position. One such water cross may be formed from four elongated metal angles that divide the fuel assemblies into four, separate, elongated fuel sections, each section having a separate, elongated fuel bundle or subassembly located therein. Each separate fuel bundle includes an upper tie plate, a lower tie plate and a plurality of elongated fuel rods disposed therebetween.
In such fuel assemblies, four elongated solid structural ribs may be centrally secured to the inner walls of the outer flow channel and spaced ninety degrees apart about the inner periphery of the outer flow channel. The lateral outer ends of the four metal angles are secured to the structural ribs to form a central water cross thereby providing a centrally disposed path for the flow of subcooled neutron moderator along the length of the fuel rods to improve the neutron moderation and economy. Such a water cross design is disclosed in commonly assigned, copending application Ser. No. 368,555 Filed Apr. 15, 1982 the disclosure of which is hereby incorporated by reference.
The above described central water cross and the four structural ribs located along the lengths of the inner walls of the outer flow channel lower the stress intensity and the creep deformation of outer flow channel by pinning its interior walls at their mid-spans, and by reducing the deformable span length by a factor of two to improve the mechanical performance of the fuel assembly. Thus, an outer flow channel having a thinner wall thickness may be used with a central cross and still have superior creep resistance and mechanical strength properties. However, such a central water cross is difficult to fashion and requires the use and fashioning of several large, precision formed metal angles.